You have 3 choices for MIDI receive handling. Your question doesn't specify which direction(s) you're concerned with (from MODX to external devices or to MODX from external devices). That said, MIDI receive is more restrictive so I will cover that first.

Mode: Multi-Channel MIDI

Here, each Part receives on the MIDI channel corresponding with the Part. Part 1 receives on MIDI channel 1. Part 2 receives on MIDI channel 2 ... through Part 16 receives on MIDI channel 16. It doesn't matter if a Part has keyboard control on or off (this will make more sense later).

Mode: Single-Channel MIDI

Here, each Part with keyboard control turned ON will receive on an assigned transmit/receive MIDI channel (single channel). Parts with keyboard control turned off will not receive.

Mode: Hybrid

This is the same as single-channel (all Parts with keyboard control ON will receive on a single transmit/receive MIDI channel) except that Parts with keyboard control OFF will receive on their corresponding Part number. So say you have 8 Parts 1-8 with keyboard control turned ON. Then Parts 1-8 will receive on a single MIDI channel you configure and then Parts 9-16 (which always has keyboard control turned off) will receive on channels 9-16 respectively.

If you want your Performance to only receive on Parts 3,4,5,6, and 14 then you would place a dummy Part in Part 1 (it cannot be empty) and then use Parts 3,4,5,6, and 14 in multi-channel MIDI mode.

MIDI transmit is a different story. It works with flexibility. Any Part can be assigned to transmit on any MIDI channel. You would use part/master zone control to change the transmit channels from the default (same as Part number when in multi-channel mode ... which is the only mode that applies because Zone control turns off/overrides single and hybrid modes).

Many thanks for that informative response.